Danbo----and you probably won't get exact answers-----Lot's of us have pondered this mystery-----myself and others here seem to agree on one thing, that those huge amounts of pressure are not usually necessary----what I started doing, is start my first press with a light weight, usually less then 1 psi and then up the weight with each flip until I get a good curd knit----it takes a little experimenting, but has worked well for me so far.
It seems the books are a little vague about exact psi, too.
Remember you also have to calculate your mechanical advantage of your cheese press. I'm pretty sure there are spread sheets and charts available on here---do a search.
Qdog